Immunity is defined as the resistance offered by the host against microorganisms or any other foreign substance(s). Immunity can be broadly classified into two types based on its specificity; less specific innate immunity, which is present right from birth, and more specific acquired or adaptive immunity acquired during life.
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Innate immunity provides the first line of defense against infection. The cellular and molecular components of innate immunity are uniform to all species and are present before the onset of infections. The innate immune system is not specific to particular pathogens, and it recognizes molecular structures unique to microbes called pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors (PRR). The innate immune system aims to eliminate microbes and other foreign particles using:
- Physical barriers such as skins
- Proteins (complement)
- Phagocytic cells (neutrophils, macrophages)
- Antimicrobial compounds synthesized by host cells
In general, our innate immune system clears most of the microorganisms before they activate the adaptive immune system.
When pathogens breach or resist the innate immune system, specific and more robust adaptive immunity comes into play to clear such pathogens. Generally, an adaptive immune response against infection is seen after 5 to 6 days of exposure to a particular antigen. Adaptive immunity is capable of recognizing and selectively eliminating specific foreign microorganisms. Unlike innate immune responses, adaptive immune responses are not the same in all members of a species. Adaptive immunity displays four characteristics attributes:
Adaptive immunity responds to the challenge with high specificity and the remarkable property of “memory.” Cells or components of the adaptive immune system can distinguish subtle differences among antigens. For example, antibodies can distinguish between two protein molecules that differ in only a single amino acid.
The immune system can generate tremendous diversity in its recognition molecules, allowing it to recognize billions of unique structures on foreign antigens.
Exposure to the same antigen in the future results in a quicker and more robust immune response because of immunological memory. Because of immunologic memory, the secondary immune response is quicker and heightened than the primary immune response, and there is a life-long memory against many infections such as measles.
The adaptive immune system typically responds only to foreign antigens, indicating that it is capable of self/nonself recognition. There are two types of adaptive immune responses, humoral or antibody-mediated immunity, and cell-mediated immunity.
Differences between Innate Immunity and Adaptive Immunity
Some of the major differences between Innate Immunity and Adaptive Immunity are summarized in the table below.
|Innate Immunity||Adaptive/Acquired Immunity|
|Definition||Innate immunity is the inborn resistance against infections that an individual possesses right from birth due to his genetic or constitutional markup.||Acquired immunity is the resistance to infecting foreign substances that an individual acquires or adapts during life.|
|Origin||Prior exposure to the antigen is not required. It is present before the first exposure to microbial antigen.||Develops during the lifetime following the antigenic exposure|
|Activity||Always present||Normally silent but triggers after exposure to pathogens|
|Diversity||Diversity is limited; It is active only against a limited repertoire of antigens.||Adaptive immunity is more varied and involves specialized immune responses.|
|Specificity||Non-specific defends against any pathogen upon first exposure||Antigen specific-responds to specific pathogen on 2nd or latter exposure|
|Functional against||General microbes (bacteria, fungi, parasites), etc., Chemical irritants, burns, tissue injury, etc.||Microbes, as well as nonmicrobial substances, called antigens|
|Response time||The immune response occurs in minutes||Takes days to generate an immune response|
|Potency||It has a limited and lower potency||It has a highly potent immune response|
|Target Antigens||Innate immunity develops against antigens that many microbes share (pathogens-associated molecular patterns, PAMPs).||Acquired immunity develops against antigens that are specific for each microbe.|
|Host Cell Receptors||Host cell receptors of innate immunity (called pattern recognition receptors) are non-specific, e.g., Toll-like receptors.||Host cell receptors are specific, e.g., T cell receptor and B cell immunoglobulin receptor.|
|Immunological Memory||Absent |
It reacts with equal potency upon repeated exposure to the same pathogen.
The presence of memory cells triggers a faster and more potent response when re-exposed to the same pathogen.
|Heritance||Innate immunity is inheritable||Passive acquired immunity is heritable from mother to neonates for a brief period after birth.|